In wireless communication systems, such as defined by 3GPP Long Term Evolution (LTE/LTE-A) specification, user equipments (UE) and base stations (eNodeB) communicate with each other by sending and receiving data carried in radio signals according to a predefined radio frame format. Typically, the radio frame format contains a sequence of radio frames, each radio frame having the same frame length with the same number of subframes. The subframes are configures to perform uplink (UL) transmission or downlink (DL) reception in different Duplexing methods. Time-division duplex (TDD) is the application of time-division multiplexing to separate transmitting and receiving radio signals. TDD has a strong advantage in the case where there is asymmetry of the uplink and downlink data rates. Seven different TDD configurations are provided in LTE/LTE-A systems to support different DL/UL traffic ratios for different frequency bands.
FIG. 1 (Prior Art) illustrates the TDD mode UL-DL configurations in an LTE/LTE-A system. Table 100 shows that each radio frame contains ten subframes, D indicates a DL subframe, U indicates an UL subframe, and S indicates a Special subframe/Switch point (SP). Each SP contains a DwPTS (Downlink pilot time slot), a GP (Guard Period), and an UpPTS (Uplink pilot time slot). DwPTS is used for normal downlink transmission and UpPTS is used for uplink channel sounding and random access. DwPTS and UpPTS are separated by GP, which is used for switching from DL to UL transmission. The length of GP needs to be large enough to allow the UE to switch to the timing advanced uplink transmission. These allocations can provide 40% to 90% DL subframes. Current UL-DL configuration is broadcasted in the system information block, i.e. SIB1. The semi-static allocation via SIB1, however, may or may not match the instantaneous traffic situation. Currently, the mechanism for adapting UL-DL allocation is based on the system information change procedure.
In 3GPP LTE Rel-11 and after, the trend of the system design shows the requirements on more flexible configuration in the network system. Based on the system load, traffic type, traffic pattern and so on, the system can dynamically adjust its parameters to further utilize the radio resource and to save the energy. One example is the support of dynamic TDD configuration, where the TDD configuration in the system may dynamically change according to the DL-UL traffic ratio. When the change better matches the instantaneous traffic situation, the system throughput will be enhanced. For example, in one scenario, multiple indoor Femto cells deployed on the same carrier frequency and multiple Macro cells deployed on an adjacent carrier frequency where all Macro cells have the same UL-DL configuration and the indoor Femto cells can adjust UL-DL configuration. In another scenario, multiple outdoor Pico cells deployed on the same carrier frequency and multiple Macro cells deployed on an adjacent carrier frequency where all Macro cells have the same UL-DL configuration and outdoor the Pico cells can adjust UL-DL configuration.
Although the system benefits from the flexible mechanism, the new mechanism also has impacts on the design of UE, such as UE measurement. In LTE, the UE measurement is an important mechanism to support cell selection or reselection, scheduling, and mobility. A measurement scheduler is designed to control the physical layer at UE to collect measurement samples, which can be the RSRP, RSRQ and/or RSSI of the reference signal. With the adaptive TDD configuration feature, the network can adjust the TDD pattern to better match the DL/UL traffic pattern. However, since TDD configuration may change frequently, UE measurement may be impacted if the TDD change is not sent to UEs in time. Therefore, the way to notify UEs on the TDD configuration change and the way of UE doing measurement are important. Otherwise, the incorrect or inaccurate measurement results may affect handover and cell reselection results.
FIG. 2 (Prior Art) illustrates an LTE/LTE-A mobile communication system 200 with adaptive TDD configuration. Mobile communication system 200 comprises a Macro base station eNB 201 serving Macro cell 1, base station eNB 202 serving small cell 2, and base station eNB 203 serving small cell 3. Cell 1 is a Macro cell and its TDD configuration is more static. Small Cells 2-3 are within the macro cell's coverage. Cell 2 and Cell 3 form an isolated cell cluster 1, where TDD configuration can be independently adjusted. All cells in an isolated cell cluster should apply the TDD configuration change together. In this example, assume cell 1 applies TDD configuration 5, which is configured semi-statically, and the isolated cell cluster, i.e. cell 2 and cell 3, originally applies TDD configuration 5. As more UL traffic is demanded in the isolated cluster, it changes the TDD configuration to TDD configuration 3.
Assuming there are four UEs in the system. UE1 is a RRC connected UE served by an adaptive TDD cell, e.g., cell 2; UE2 is a RRC idle UE camped on the other adaptive TDD cell, e.g., cell 3; UE3 is a neighbor cell RRC connected UE served by the macro cell, e.g., cell 1; and UE4 is a neighbor cell RRC idle UE camped on the macro cell, e.g., cell 1. The UEs may or may not know the TDD configuration change in the isolated cell cluster and thus the measurement results on the cells in the cluster may be influenced. For example, the TDD configuration change notification may be sent through a dedicated signaling, e.g., RRC signaling or MAC signaling or PDCCH signaling. As a result, UE1 may know the new TDD configuration pattern while UE2, UE3, and UE4 may not know this information due to not receiving the dedicated signaling.
In a first scenario, an RRC connected mode UE (e.g., UE1) can successfully receive the TDD change notification so that it can measure on the exact DL subframes based on the notification.
In a second scenario, since the TDD change is notified by the dedicated signaling, an RRC idle mode UE (e.g., UE2) will not receive the notification. The measurement of UE2 will be impacted. Assume UE2 originally measures on subframes #0, #4, #8, which are DL subframes in TDD configuration 5 (depicted by box 101 in FIG. 1). However, subframe #4 is adjusted to UL subframe after the isolated cell cluster adopts TDD configuration 3 (depicted by box 102 in FIG. 1). When UE2 continue to measure subframe #4, it may lead to false alarm measurement if the power level of RBs carrying reference signal is large, thus affects UE2 doing cell reselection.
In a third scenario, a neighbor cell UE (e.g., UE3) who attempts to handover to the isolated cell cluster and performs the handover measurement may also be affected. If the adaptive TDD configuration is not real time updated to the neighbor cell, then the neighbor UE may do the measurement in the incorrect subframe and it may lead to incorrect measurement results. For example, UE3 moves towards cell 2 and performs measurement on cell 2. UE3 should measure Cell 2 in it measurement gap. Assume UE3 measures subframe #0, #3, #6, and #9 of cell 2 originally. If cell 2 changes to TDD configuration 3 without immediately updating to its neighbor cell, then UE3 in the neighbor cell may wrongly estimate cell 2's link quality because UE3 still measures subframe #3. Therefore, the handover event may be triggered at incorrect timing and the handover performance may degrade.
In a fourth scenario, UE4 is an RRC idle UE in the neighbor cell. Thus, UE4 may not know the instantaneous TDD change of its measured neighbor cell. The incorrect measurement on the neighbor cell may lead to incorrect cell reselection judgment.
A solution is sought.